1. Field of the Invention
The present invention generally relates to electrode materials for batteries and, more particularly, to an inexpensive method of preparing Li.sub.x Mn.sub.2 O.sub.4 for lithium-ion batteries.
2. Description of Related Art
The use of lithium-ion batteries has continued to increase for many applications. LiCoO.sub.2 has been used extensively as an intercalation cathode material in lithium-ion batteries. But due, in part, to the high cost of production and the toxicity of LiCoO.sub.2, spinel structure Li.sub.x Mn.sub.2 O.sub.4 has become increasingly considered as an alternate intercalation cathode material for secondary lithium-ion batteries. The use of Li.sub.x Mn.sub.2 O.sub.4 is advantageous not only because it is non-toxic but also due to its performance characteristics, including reliability, long cycle life, high cell capacity, high energy density, and safety.
The performance characteristics of Li.sub.x Mn.sub.2 O.sub.4 are important for a wide range of uses, such as in cellular telephones, military devices, automobiles, and even space satellites. However, as the scale of usage and volume of production increases, the time and cost involved in producing the Li.sub.x Mn.sub.2 O.sub.4 becomes of increasing significance.
Some of the past processes for preparing lithium maganese oxides have been based on what is referred to as the "Pechini process." That process is described by Pechini in U.S. Pat. No. 3,330,697 whereby alkaline earth titanates, zirconates and niobates are prepared using alpha-hydroxycarboxylic acids, such as citric and lactic acids. Polybasic acid chelates are formed between those acids and titanium, zirconium, and niobium. The chelates then undergo esterification when heated with a polyhydroxyl alcohol. Thereafter, the excess polyhydroxyl alcohol is removed.
An example of using the Pechini process to make spinel Li.sub.x Mn.sub.2 O.sub.4 is shown by Liu et al. in "Synthesis and Electrochemical Studies of Spinel Phase Li.sub.x Mn.sub.2 O.sub.4 Cathode Materials Prepared by the Pechini Process" Journal of the Electrochemical Society, Vol. 143, No. 3, pp. 879-884 (1996). Liu et al. point out that, in the past, Li.sub.x Mn.sub.2 O.sub.4 has been synthesized by the mixing of oxides and/or carbonates followed by high temperature firing and extended grinding. However, as further pointed out by Liu et al., those conditions may result in nonhomogeneity, abnormal grain growth, and poor control of stoichiometry. Therefore, Liu et al. dissolved metal nitrates in a mixture of citric acid and ethylene glycol. A clear solution was then produced by heating the mixture. The clear solution was further heated at a higher temperature to induce esterification and distill out excess ethylene glycol, which resulted in a viscous solution. That viscous solution was vacuum dried to yield a polymer foam. The foam was calcined at temperatures between 250 to 800 degrees C. for a few hours.
Another method for preparing lithium oxides is described in U.S. Pat. No. 5,211,933 wherein a sol-gel process is utilized. Manganese or cobalt acetates are hydrolized with lithium and ammonium hydroxides. Thereby, a gelatinous precipitate is formed and then heated to form a xerogel. And in U.S. Pat. No. 5,266,299, a method of preparing a lithium manganese oxide includes reacting LiMn.sub.2 O.sub.4 with Lil in an evacuated container, heating it, and then washing it to remove unreacted Lil. Alternatively, the same mixture could be refluxed in acetonitrile, filtered, and then washed.
As can be seen, there is a need for improved methods of preparing electrode materials for batteries and, in particular, an inexpensive method of preparing Li.sub.x Mn.sub.2 O.sub.4 for lithium-ion batteries that reduce the number of processing steps and other required chemicals to thereby reduce the overall production costs.